Point your sister at http://www.novadevelopment.com where she will find something like she is looking for.
Also, check out the Apple web site by clicking on the Apple (top left of screen) and selecting Mac OS X Software. There are thousands of titles up there so she will have to search.
Google also works well. Try "Greetings card programs for os x" and see what you get. WIth or without quotes
My sister is looking for a program to make greeting cards and possibly calenders. I told her I'd see if I could find something for her. She is a recent switcher and doesn't know where to find mac stuff. She has Photoshop Elements but that is a little too much for her. She's looking for something a little easier.
Thanks for any suggestions.